The present disclosure relates roll bending machines having three or four rolls, which are well known in the metal fabricating industry for rolling metal plate into cylinders, obrounds and cone shapes. A traditional roll bending machine 100 is shown in FIG. 1, which illustrates generally known arrangements for determining the position of hydraulic cylinders that operate to adjust a distance between bending rolls to, thus, adjust the bend radius of a bent plate. More specifically, the machine shown in FIG. 1 includes a rectilinear displacement transducer 101 associated with a hydraulic actuator 102 and associated with a moveable roller 103. The machine further includes string encoders 105 and 105′ that are associated with respective hydraulic cylinders 106, 106′ that are arranged to set or adjust a position of the machine's adjustable rolls 107 (only one visible). A rectilinear displacement transducer is a potentiometer that provides a voltage representative of position such that, based on the voltage from the transducer, a controller can infer or estimate the position of the hydraulic cylinder and, thus, the distance between rolls.
As is known, the output voltage of a rectilinear displacement transducer sensor will change with wear and scratches. In the particular application of a roll bending machine, lubricants and contaminants in the operating environment of the machine may also find their way into the sensor and, particularly, along a sliding interface between a resistive strip and a sensor tip, which will further degrade sensor life, accuracy and performance. Wear and scratches can also be caused by shock and vibration caused by rolling steel plates. Temperature is also a factor that can affect sensor performance and accuracy. Though contained within a housing and somewhat protected by mounting position, rectilinear displacement transducers used in roll bending machine applications are known to be prone to damage.
Similarly, string encoders, which are often also referred to as pull wire or pull rope encoders, are known to be relatively more reliable than potentiometer-based transducers in that they use a string 110 or 110′ that can deflect or bend out of the way of a falling object. However, these strings are necessarily exposed such that a falling object hitting the string can cause a temporary change in feedback information and cause unwanted movement of the roll, which may cause a defect in the part being rolled. Resolution of string encoders is such that rolls can be leveled or set parallel to the gripping rolls within a tolerance of 1 mm.